1. Field of the Invention
The present invention relates to a catalyst for cleaning exhaust gas and manufacturing method thereof, and in p particular, to a catalyst for cleaning exhaust gas with excellent low-temperature and high-temperature cleaning performance as well as excellent heat resistance, which can simultaneously remove HC, CO and NOx in exhaust gas discharged from automotive engines.
2. Description of the Prior Art
In general, because the exhaust gas discharged from automotive engines contains atmospheric pollutants such as HC (hydrocarbon), CO (carbon monoxide), NOx (nitrogen oxides), etc., regulations on exhaust gas has been strengthened worldwide in recent years. In particular, in the State of California, USA where air pollution causes serious social problems, the corporate average NMOG regulations have been introduced and the regulation level of the exhaust gas has been strengthened stepwise, but in order to cope with these regulations, low-pollution vehicles such as LEV or ULEV must be introduced into the market (to be introduced stepwise from year 1997 to year 2000).
In this way, to the exhaust system of automotive engines, generally, an exhaust gas purification device (catalyst converter) using catalyst for purifying exhaust gas is installed for purifying exhaust gas. For such catalysts for purifying exhaust gas, conventionally, noble metal catalysts such as platinum, rhodium, etc. supported by a porous catalyst substrate such as alumina, etc. have been popularly adopted.
Now, the exhaust gas purifying catalysts using noble metal catalysts such as platinum, rhodium, etc. have drawbacks of low catalytic activity at low temperature. Consequently, when exhaust gas temperature is low, for example, right after the engine begins to start, there is a problem that the engine emission performance is degraded because temperature of the exhaust gas purifying catalyst is not sufficiently high. Therefore, in recent years, exhaust gas purifying catalysts using palladium with high catalytic activity at low temperature for a catalytic component have been proposed (for example, see Japanese Non-examined Patent Publication No. Hei 5-184876, Japanese Patent Publication No. Hei 4-72577, and Japanese Non-examined Patent Publication No. Hei 1-281144).
However, conventional exhaust gas purifying catalyst using palladium as a catalytic component provides a high catalytic activity at low temperature but has problems in that catalytic activity of CO or NOx at low temperature is low and in that NOx purifying performance at high temperature is low. Particularly, in recent years, in automotive engines, the exhaust gas purifying device tends to be located in the upstream portion of the exhaust gas passage, in order to rapidly increase temperature of the exhaust gas purifying catalyst after the engine begins to start. And locating the exhaust gas purifying device in the upstream portion of the exhaust passage in this way causes the temperature of the exhaust gas purifying catalyst to increase under normal condition, giving a rise to a problem in that the NOx purifying performance becomes even lower in particular at high temperature.